Because of the good tensile strength, toughness, impact resistance, gas barrier property, and weather and chemical resistance, polyamide (also known as Nylon) is widely used in various industries, such as the automotive, textile and electrical industries, and can be used in a broad range of new applications. For example, polyamide filament can be made into socks, underwear lining, sweatshirts, and ski raincoats. The fabric produced by blending the staple polyamide fiber with cotton and viscose fibers can be imparted with good wear resistance and strength. Polyamide fibers may also be used in velcro, carpet, cord fabric, conveyor belts, fishing net, cable, canvas roofs or the like. Aromatic polyamides are particularly heat-resistant and fire-resistant, and are applicable to special fire-protection and high-tech materials.
In regard to the methods for producing amides or polyamides, TW 200800867 and TW 200906912 developed a new polycondensation synthesis mechanism, i.e., sequential self-repetitive reaction, in which a diisocyanate and a diacid monomer are thermally condensed in the presence of a CDI catalyst at an elevated temperature to produce a polyamide. The sequential self-repetitive reaction at least comprises the following three repetitive and sequential reaction steps:                (1) in the presence of a CDI catalyst, such as 1,3-dimethyl-3-phospolene oxide (DMPO) or 3-methyl-phenyl-3-phosphorene-1-oxide (MPPO), condensing two molecules of an isocyanate, to synthesize an aromatic carbodiimide (CDI);        (2) adding a carboxylic acid to the aromatic carbodiimide, to form a reaction intermediate N-acyl urea; and        (3) thermally cracking the N-acyl urea, to produce a polyamide and one molecule of isocyanate,where the isocyanate produced in Step (3) proceeds to Step (1) so as to conduct a new round of sequential self-repetitive reaction for synthesizing the polyamide, until all the isocyanate is completely consumed.        
Isocyanates are highly reactive so they easily react with water to produce urea. In addition, isocyanates have other disadvantages such as difficulty in storage, high toxicity and tendency for side reactions. Therefore, isocyanates have limitations and defects when used in a sequential self-repetitive catalyzed thermal reaction. Moreover, the inventors of the present invention have found that conventional high-polarity polymerization solvents containing tertiary amine nitrogen, such as anhydrous N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc), and N,N-dimethylformamide (DMF), that are commonly used for preparing amides or polyamides are prone to react with the isocyanate to produce by-products. It is further found that in most cases, non-linear and non-amide by-products could be formed and result in a bimodal distribution of the prepared polyamide when analyzed for the molecular weight distribution by GPC. The above result shows that the prepared polyamide is accompanied by obvious by-products, so the physical and mechanical properties of the prepared product are less desirable and more inclining to be degraded. Accordingly, to solve the technical problem associated with isocyanate being prone to react with water, the present invention suggests using an aromatic carbamate, rather than an isocyanate, as an alternative starting material of a catalyzed thermal reaction. By using an aromatic carbamate, water could be first removed during the initial thermal treatment. The de-watered aromatic carbamate is then decomposed into an isocyanate at an elevated temperature. Hence, the side reaction is suppressed so as to improve the process conditions. In this way, in comparison with the prior art process, a polyamide produced according to the method of the present invention could have improved physical and mechanical properties.
Another object of the present invention is to provide an improved method for preparing an aromatic carbamate that is used as a stable material for producing isocyanate in the present invention. At the time of the priority date of the present invention, the phosgene process is still a major process in the industrial production of isocyanate. However, the phosgene process incurs strong toxicity. For example, a large amount of by-product hydrogen chloride is produced during a phosgene process. In addition, since the product has relatively high chlorine content, the production equipment is expensive and requires frequent maintenance. Furthermore, the by-product hydrogen chloride produced during the phosgene process is a corrosive compound that easily reacts with an amine compound to form a solid amine salt. Although the amine salt may be further reacted with phosgene to produce the isocyanate, the reaction rate is slow and the reaction needs to be conducted at a high temperature. Therefore, once the solid amine salt is deposited in a pipeline, public safety incidents may be caused due to the obstruction of the corroded pipeline. As such, considering the environmental protection and public safety issues encountered during a phosgene process, there is still a need for an efficient and green method to replace such process.
The preparation of an aromatic carbamate through the diphenyl carbonate (DPC) process was first proposed by N. Yamazaki in 1979 (N. Yamazaki, T. Iguchi, Journal of Polymer Science: Polymer Chemistry Edition, 17, 835, 1979). However, the selectivity and yield of the aromatic carbamate product prepared according to the process are at most 80%, and the reaction rate is quite slow, so when the reaction temperature is elevated, the content of the by-product urea is increased accordingly. U.S. Pat. Nos. 6,143,917, 6,781,010 and EP 2275405 also developed processes for preparing an aromatic carbamate with diphenyl carbonate (DPC), so as to produce an isocyanate. Considering the insufficient selectivity and yield of the aromatic carbamate prepared according to conventional diphenyl carbonate processes, another objective of the present invention is to increase the yield of the aromatic carbamate to 90% or higher, preferably 95% or higher, and particularly preferably 98% or higher, by replacing the prior art with a novel combination of catalysts.